Radar systems for measuring a distance, relative speed, and angle of objects (e.g., of vehicles, obstacles, etc.) are increasingly being used for safety and convenience functions in motor vehicles. So-called multiple-input multiple-output (MIMO) systems, in which multiple transmitting and receiving antennas are used, are increasingly being utilized. Particularly accurate angle estimates can be made with the aid of the MIMO principle, the antenna aperture (antenna area), which is important for angle estimation, being virtually enlarged. Here multiple transmitting antennas emit their signals without mutual influence, the signals being divided among the reception channels. The virtual enlargement of the aperture is achieved by the fact that the spacing of the transmitting antennas is different from the receiving antennas; it is thus possible to proceed computationally as if only a single transmitting antenna were present, but the number of receiving antennas is multiplied and what results virtually is a greater width and/or height of the antenna aperture.
Separation of the signals of the various transmitting antennas can be accomplished in the frequency domain or time domain. The separation is often effected in the time domain, i.e. the antennas transmit successively on a time division multiplexed (TDM) basis. A disadvantage here is that the measurement time increases due to the sequential measurement, and objects may have moved appreciably during the extended measurement time, which can decrease measurement accuracy.
Another separation possibility is separation in the frequency domain (frequency multiplexing). Here different antennas occupy different frequency ranges at the same point in time. A disadvantage of this method is the reduced available bandwidth for each transmission channel. The distance separation capability of a radar system is directly proportional to its bandwidth, with the result that the distance separation capability can be decreased with conventional frequency multiplexing.
The statements above apply regardless of the modulation method used. Typical transmission frequencies nowadays are 24 GHz or 77 GHz; maximum bandwidths that can be occupied are less than approx. 4 GHz, but typically appreciably less, for example approx. 0.5 GHz.
Present-day motor vehicle radar systems generally use FMCW modulation, in which multiple linear frequency ramps of different slopes are successively cycled through. Mixing the instantaneous transmitted signal with the received signal yields a low-frequency signal whose frequency is proportional to distance but which also contains an additive/subtractive component thanks to a Doppler frequency that is proportional to the relative speed.
Separation of distance and speed information for multiple targets is accomplished with a complicated and relatively error-prone method in which the results of the various ramps are combined with the results of measurements performed earlier.
More recent systems use FMCW modulation with considerably faster ramps (chirp modulation), with the result that the Doppler shift within a ramp becomes negligible. The distance information obtained therefrom is largely unequivocal, and a Doppler shift can then be determined by observing the development over time of the phase of the complex distance signal.
In the future, digital modulation methods will also play an important role in motor vehicle radar systems. Digital modulation methods such as orthogonal frequency division multiplexing (OFDM) are already being used in some communications applications (e.g. WLAN, LTE, DVB-T).